Cloud computing means the machines, disks, and databases live in a data centre you rent, and you turn them on through an interface instead of driving to an office to plug in a server. You pay for what you leave running. You can add capacity without buying a box. You can also leave a forgotten machine running and discover it on the bill. The cloud is a supply of infrastructure. It is not a redesign of your application.
A rented room is not a new building
The common failure is to take the server from under the desk, copy it onto a virtual machine, and call the afternoon a migration. The backup is still a habit. The disk still fills. The only new fact is an invoice. Real use of the cloud is boring and specific: a database you can restore because you have restored it, identities that are not a shared password, and a way to turn off what you do not need.
It is also not "everything must move". The accounts package can stay where finance already trusts it. The file share the floor uses can move with a path the PCs already know. The thing that must not have a single copy under a desk is the thing you move first.
What you are actually buying
You are buying someone else's building, power, and network, plus tools to create resources and throw them away. You are not buying an excuse to skip a backup test. You are not buying automatic speed. A slow query is still a slow query on a larger machine, until the bill arrives and the query is still slow.
Leaving should be thinkable. Export the database. Know which services are yours and which are the vendor's special sauce. A move you cannot reverse is a gamble with a monthly fee.
When it matters on a project
It matters when the office server is the only copy, the power fails, or the disk is a rumour. It matters when a proper backup and a second site are cheaper than another tower under the desk. It does not matter as a slogan on a rewrite. Do not rebuild the software and change data centres in the same week. You will not know which change broke the number.
That separation is the work in legacy and cloud modernisation: what moves, what stays, and a restore you have proved. The application still has to be sound. The cloud will not repair a system that cannot tell you yesterday's stock.